1. Introduction

According to the US Food and Drug Administration (FDA), a cosmetic is defined as “a product (excluding pure soap) intended to be applied to the human body for cleansing, beautifying, promoting attractiveness, or altering the appearance” ^[1]. This definition applies to products for skin, hair, and oral care. It is important to note that this description does not include any health claims.

The increased interest in microbes colonizing the human body, not simply those infecting it, has led to many studies attempting to manipulate the microbiome in a given niche, in favour of health. The use of beneficial microbes for this purpose has seen the field of probiotics grow substantially. Defined as “live microorganisms that, when administered in adequate amounts, confer a health benefit on the host,” probiotic applications range widely in type, scope, and application ^[2]. This includes cosmetic applications, where the market for probiotics is projected to grow at a 12% rate in the next ten years, with North America the driver ^[3].

2. Cosmetics for Skin

The increase in products termed probiotic on the market does not necessarily equate with a reason to celebrate the successful translation of science to commerce and consumers. Too many products fail to comply with the characteristics required to be called probiotic. Many false claims and rampant misuse of the term has resulted in mainstream consumer channels providing incorrect information to consumers. Probiotics are not inside us, not in fermented food, not necessarily better if there are more species or a higher viable count. Formulations are being concocted not based on research evidence but on marketing and what might appeal to consumers. For example, products are being composed supposedly to improve vaginal health using strains not documented to compete with urogenital pathogens, improve immunity or do anything that can restore homeostasis to that region of the body. In other words, there are no data to support their selection. Yet, the internet, the use of words to reach the first page of search engines, and use of pseudo-experts for promotion provide a means for these products to be highly rated and appear to be the best clinically documented for preventing or curing bacterial or yeast infections in the vagina.

The net result is misleading and confusing to consumers, as well as making healthcare professionals leery of the whole field of probiotics. To counter this, we need to re-state important facts.

For a product to be considered probiotic, it must comply with three core characteristics: 1. The strain(s) must be characterized, including genetically and phenotypically and a rationale given based on documented experiments published in peer-reviewed papers, for their inclusion for the intended use. 2. The product must contain sufficient live microorganisms at the time of use that are equivalent to when the product was shown in clinical studies to confer a benefit to the desired target site. 3. The delivery method, dosage, and duration of use should be based on scientific evidence in humans if humans are the intended recipient.

It makes it difficult for potential users if the product label does not state strain designations because it becomes impossible to track the research performed on the contents. Dosages are rarely stated on labels, and some products only contain filtered extracts or ferments or lysed bacteria, meaning that no live microorganisms are present: thus, the product is not probiotic and the term should not have been used.

The cosmetic industry has ventured into this space by focusing its efforts towards skincare. There are several areas of opportunity and great value in this concept.

A search of the websites of two major retailers of cosmetics in North America revealed that at least 50 products are already being commercialized with a claim to contain probiotics ^[4][5]. Figure 1 shows a word cloud with the top 30 terms used in their statements. The majority of them are targeted for skincare, although some are for deodorants and hair care. The most common claims are geared towards “balancing” the skin microbiome, improving the skin barrier, and enhancing the skin’s overall appearance. Table 1 breaks down the types of products included in this analysis, and Table 2 includes the ingredients as listed on their labels; all of them correspond to different types of products targeted for skincare. All products included in this analysis can be used by any gender.

Table 2. Cosmetic products and their ingredients. Data obtained from 50 products advertised by major retailers of cosmetics. * The ingredient listed as “Bifida ferment lysate” corresponds to a lysate from Bifidobacterium longum reuter.

These claims by themselves require critiquing. There is no single healthy skin microbiome, so what would it take to ”balance” the one that a given individual possessed? Furthermore, the skin has many layers, with microbes being detected in the dermis, adipose, follicle, epidermis ^[8]. A product claiming to “balance” the microbiome should have studies indicating in many subjects, preferably hundreds, how a given probiotic product changes the various layers of microbiota in such a way as to restore and maintain it to what is deemed healthy for each individual. Since it seems highly unlikely that such studies have been performed for most if any products, claims of balancing the skin microbiome should not be made. This is important because consumers like the sound of products that do that, especially since terms like the “microbiome” and “balance” are being used so widely.

Enhancement of the skin’s overall appearance can be subjective for consumers, but it also has some scientific principles. Factors such as the reduction of the contrast, presence of visible ageing marks or spots, skin colour, melanin, and hemoglobin can be measured ^[9]. This and other assessments provide the means to give tangible results that can then provide substance to claims of improvements.

The ability of certain probiotic strains to improve epithelial and epidermal barrier function has been reported. The latter is so critical in the function of the skin and a well-used target for making claims that appeal to consumers. If strains being used as cosmetics do indeed improve barrier function, experiments can be performed to verify this. Indeed, researchers from a well-renowned cosmetic company have shown that a lysate from the probiotic Bifidobacterium longum reuter strain could decrease vasodilation, edema, mast cell degranulation, and TNF-alpha release, and using trans-epidermal water loss to assess barrier function, showed improvement with application of the lysate containing product ^[10]. Some products state that they include a filtrate of either ferments or lysates. In the case of filtrates, bacterial cells (alive or not) are removed along with potentially some other larger weight molecules (e.g., peptides). This might remove some of the bioactive compounds of the preparation and components of the bacterial cells required to observe specific benefits. Therefore, filtrates are excluded from the definition of postbiotics and cannot be deemed to be probiotic.

Lysates are cells whose outer membrane has been broken down due to chemical or physical processes ^[11]. These preparations have been used in medical practice as immunomodulators for fifty years. They can contain bacterial components that up-regulate the immune response of the host cells; they are particularly effective in the management of infections of the respiratory tract ^[12]. Lysates of Lacticaseibacillus rhamnosus GG and B. longum can increase tight-junction barrier resistance in vitro by modulating protein components ^[13]. Although there is value in using these types of preparations, further studies are still required on a strain-dependent basis before drawing conclusions and making claims. The cell composition, elasticity, and activation of macrophages differ between bacterial strains, even within the same species ^[14]. In one study, lactobacilli lysates altered their ability to increase re-epithelialization of keratinocytes ^[15], again emphasizing the need to check strain properties prior to making claims.

Intriguingly, little has been reported on the chemical composition of the lysates being used in cosmetics. This should require cell wall and analysis, including the use of liquid chromatography–tandem mass spectrometry (LC-MS/MS)-based metabolomics ^[16] or newer methods such as surface-enhanced Raman spectroscopy (SERS) ^[17]. In doing so, it will soon be extremely apparent that metabolite types and quantities differ between strains, and therefore their application to human tissues would also differ. This again illustrates the need to perform tests in humans with whole microbial cells, lysates, or filtrates to show what activity is being promoted by the application of any given product; ideally to know which component of the lysate is responsible.

There are numerous studies providing evidence of the benefits of specific probiotic strains for skin health ^[18][19][20]. In addition, the mechanisms of anti-ageing suggest strains can help to regulate pH, reduce oxidative stress, protect from photoaging, and improve the skin barrier function ^[21].

However, the cosmetic industry needs to be consistent and transparent in its labelling practices and direct efforts to generating more scientific evidence before making claims.